Potent agonists of growth hormone‐releasing hormone: Part I

Analogs of the 29 amino acid sequence of growth hormone-releasing hormone (GH-RH) with agmatine (Agm) in position 29 have been synthesized by the solid phase method, purified, and tested in vitro and in vivo. The majority of the analogs contained desaminotyrosine (Dat) in position 1, but a few of them had Tyr¹, or N-MeTyr¹. Some peptides contained one or more additional L- or D-amino acid substitutions in positions 2, 12, 15, 21, 27, and/or 28. Compared to the natural sequence of GH-RH(1-29)NH₂,[Dat¹,Ala¹⁵]GH-RH(1-28)Agm (MZ-3-191) and [D-Ala²,Ala¹⁵]GH-RH(1-28)Agm (MZ-3-201) were 8.2 and 7.1 times more potent in vitro, respectively. These two peptides contained Met²⁷. Their Nle²⁷ analogs, [Dat¹,Ala¹⁵,Nle²⁷]GH-RH(1-28)Agm (MZ-2-51), prepared previously (9), and [D-Ala²,Ala¹⁵,Nle²⁸]GH-RH(1-28)Agm (MZ-3-195) showed relative in vitro potencies of 10.5 and 2.4, respectively. These data indicate that replacement of Met²⁷ by Nle²⁷ enhanced the GH-releasing activity of the analog when the molecule contained Dat¹-Ala² residues at the N-terminus, but peptides containing Tyr¹-D-Ala² in addition to Nle²⁷ showed decreased potencies. Replacement of Ser²⁸ with Asp in multi-substituted analogs of GH-RH(1-28)Agm resulted in a decrease in in vitro potencies compared to the parent compound. Thus, the Ser²⁸-containing MZ-2-51, and [Dat¹-Ala¹⁵,D-Lys²¹,Nle²⁷]GH-RH(1-28)Agm, its Asp²⁸ homolog (MZ-3-149), possessed relative activities of 10.5 and 5.6, respectively. In vivo after the iv injection, the analogs [Dat¹,Ala¹⁵,Nle²⁷,Asp²⁸]GH-RH(1-28)Agm (MZ-3-149), [Dat¹,Ala¹⁵]GH-RH(1-28)Agm, (MZ-3-191) and [D-Ala²,Ala¹⁵]GH-RH(1-28)Agm (MZ-3-201) showed a potency equivalent to 7.6, 4.9 and 3.3 times that of GH-RH(1-29)NH₂, respectively, at 5 min and 203, 4.3 and 1.7 times higher, respectively, at 15 min. After sc administration, analogs MZ-3-149, MZ-3-191, and MZ-3-201 were shown to be 63.7, 55.2 and 56.8 times more potent than the parent hormone at 15 min and 57.6, 60.6, and 42.6 times more active, respectively, at 30 min. In addition, MZ-3-149 had prolonged GH-releasing activity as compared to the standard, and proved to be more potent than MZ-2-51, the most active member of our previous series (8, 9). Our studies indicate that very potent GH-RH analogs can result from the combination of agmatine in position 29 with other substitutions.